Written and peer-reviewed by physicians—but use at your own risk. Read our disclaimer.

banner image


Trusted medical answers—in seconds.

Get access to 1,000+ medical articles with instant search
and clinical tools.

Try free for 5 days

Atrioventricular block

Last updated: August 2, 2020


Atrioventricular block (AV block) is characterized by an interrupted or delayed conduction between the atria and the ventricles. AV blocks are divided into three different degrees depending on the extent of the delay or interruption. First-degree blocks are identifiable on ECG by a prolonged PR interval. Most patients with first-degree block are asymptomatic, and the condition is usually an incidental finding. Second-degree AV blocks are further divided into two different subtypes. Mobitz type I, or Wenckebach, blocks exhibit a progressive prolongation of the PR interval that culminates in a non-conducted P wave (“dropped beat”). Most patients with Mobitz type I blocks are asymptomatic. The Mobitz type II block generates dropped QRS complexes at regular intervals (e.g. 2:1, 3:1, or 3:2), often leading to bradycardia. Symptoms of patients with Mobitz type II block range from fatigue to dyspnea, chest pain, and/or syncope. This fixed second-degree block frequently progresses to a third-degree block. A third-degree AV block involves total interruption of the electrical impulse between the atria and ventricles. The complete absence of conduction results in a ventricular escape mechanism, which may be dangerously slow and result in life-threatening bradycardia or Stokes-Adams attacks. Therefore, a third-degree AV block is an absolute indication for pacemaker placement.



First-degree AV block



  • May be found in healthy individuals, e.g., in athletes with vagal tone
  • Usually asymptomatic
  • Often discovered incidentally on ECG


  • Clinical assessment for underlying diseases (e.g., structural heart diseases, electrolyte imbalances)
  • Usually no specific treatment necessary
  • Follow-ups to evaluate progression of the disease
  • Pacemaker


Second-degree AV block

Mobitz type I/Wenckebach


  • Progressive lengthening of the PR interval until a beat is dropped; regular atrial impulse does not reach the ventricles (a normal P wave is not followed by a QRS-complex)
  • Rate of SA node > heart rate; mostly regular rhythm separated by short pauses, which may lead to bradycardia

Symptoms/clinical findings


  • Asymptomatic patients
  • Symptomatic patients
    • Hemodynamically stable
      • Monitoring with transcutaneous pacing pads
      • If symptoms not reversible → placement of a permanent pacemaker
    • Hemodynamically unstable

Mobitz type II


  • Single or intermittent non-conducted P waves without QRS complexes
  • The PR interval remains constant.
  • The conduction of atrial impulses to the ventricles follows regular patterns:
    • 2:1 block: regular AV block that inhibits conduction of every other atrial depolarization (P wave) to the ventricles (heart rate = ½ SA node rate)
    • 3:1 block: regular AV block with 3 atrial depolarizations but only 1 atrial impulse that reach the ventricles (heart rate = ⅓ SA node rate)
    • 3:2 block: regular AV block with 3 atrial depolarizations but only 2 atrial impulses that reach the ventricles (heart rate = ⅔ SA node rate)

Symptoms/clinical findings


  • Hemodynamically stable patients:
  • Hemodynamically unstable patients:

The second-degree AV block Mobitz type II may progress to a third-degree block and is an unstable condition that requires monitoring and treatment!


Third-degree AV block (complete heart block)


Symptoms/clinical findings

Symptoms depend on:




  1. Sauer WH. First degree atrioventricular block. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/first-degree-atrioventricular-block.Last updated: November 9, 2015. Accessed: February 19, 2017.
  2. Sauer WH. Second degree atrioventricular block: Mobitz type I (Wenckebach block). In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/second-degree-atrioventricular-block-mobitz-type-i-wenckebach-block.Last updated: September 15, 2016. Accessed: February 19, 2017.
  3. Alaeddini J. First-Degree Atrioventricular Block. First-Degree Atrioventricular Block. New York, NY: WebMD. http://emedicine.medscape.com/article/161829. Updated: December 30, 2015. Accessed: February 19, 2017.
  4. Le T, Bhushan V, Chen V, King M. First Aid for the USMLE Step 2 CK. McGraw-Hill Education ; 2015
  5. Sovari AA. Second-Degree Atrioventricular Block. Second-Degree Atrioventricular Block. New York, NY: WebMD. http://emedicine.medscape.com/article/161919. Updated: January 26, 2017. Accessed: February 19, 2017.
  6. Sauer WH. Second degree atrioventricular block: Mobitz type II. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/second-degree-atrioventricular-block-mobitz-type-ii.Last updated: September 15, 2016. Accessed: February 19, 2017.
  7. Sauer WH. Third degree (complete) atrioventricular block. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. http://www.uptodate.com/contents/third-degree-complete-atrioventricular-block.Last updated: August 10, 2015. Accessed: February 19, 2017.
  8. Harbison J, Newton JL, Seifer C, Kenny RA. Stokes Adams attacks and cardiovascular syncope. Lancet. 2002; 359 (9301): p.158–160. doi: 10.1016/S0140-6736(02)07376-2 . | Open in Read by QxMD
  9. Sauer WH. Etiology of atrioventricular block. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. http://www.uptodate.com/contents/etiology-of-atrioventricular-block.Last updated: January 5, 2017. Accessed: March 3, 2017.
  10. Herold G. Internal Medicine. Herold G ; 2014
  11. Dietel M, Suttorp N, Zeitz M, et al.. Harrisons Innere Medizin (2 Bände). ABW Wissenschaftsverlagsgesellschaft (2005) ; 2005
  12. Marino BS, Fine KS. Blueprints Pediatrics. Lippincott Williams & Wilkins ; 2009
  13. Jameson JL, Fauci AS, Kasper DL, Hauser SL, Longo DL, Loscalzo J. Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2). McGraw-Hill Education / Medical ; 2018
  14. Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. J Am Coll Cardiol. 2019; 74 (7): p.e51-e156. doi: 10.1016/j.jacc.2018.10.044 . | Open in Read by QxMD
  15. Burri H, Dayal N. Acute management of bradycardia in the emergency setting. Cardiovascular Medicine. 2018; 21 (04): p.98-104. doi: 10.4414/cvm.2018.00554 . | Open in Read by QxMD
  16. Murphy JG, Lloyd MA. Mayo Clinic Cardiology Concise Textbook and Mayo Clinic Cardiology Board Review Questions & Answers. CRC Press ; 2007
  17. Olshansky B, Chung MK, Pogwizd SM, Goldschlager N. Arrhythmia Essentials E-Book. Elsevier Health Sciences ; 2016
  18. Kusumoto FM, Goldschlager NF. Cardiac Pacing for the Clinician. Springer Science & Business Media ; 2007
  19. Ferri FF. Ferri's Clinical Advisor 2012. Elsevier Health Sciences ; 2011
  20. Wittich, CM. Mayo Clinic Internal Medicine Board Review. Oxford University Press ; 2019
  21. Olshansky B, Chung MK, Pogwizd SM, Goldschlager N. Arrhythmia Essentials. Jones & Bartlett Publishers ; 2011
  22. Abdullah A. ECG in Medical Practice. JP Medical Ltd ; 2014
  23. Aronow WS, Fleg JL, Rich MW. Tresch and Aronow's Cardiovascular Disease in the Elderly. CRC Press ; 2019
  24. Deedwania P, Raviele A. Clinical and Electrophysiologic Management of Syncope, an Issue of Cardiology Clinics. Elsevier Health Sciences ; 2015
  25. Ziad I; Mille J. Clinical Arrhythmology and Electrophysiology: A Companion to Braunwald's Heart Disease. Saunders ; 2008
  26. Cho SW, Kang YJ, Kim TH, et al. Primary Cardiac Lymphoma Presenting With Atrioventricular Block. Korean Circulation Journal. 2010; 40 (2): p.94. doi: 10.4070/kcj.2010.40.2.94 . | Open in Read by QxMD
  27. Eagle KA, Baliga RR. Practical Cardiology. Lippincott Williams & Wilkins ; 2008
  28. Heger JW, Niemann JT, Criley JM. Cardiology. Lippincott Williams & Wilkins ; 2004